Method and apparatus for transmitting a RACH preamble on a wireless network

1. A method in a user equipment, the method comprising:

receiving a first configuration of a first set of random access channel occasions, where the first configuration of the first set of random access channel occasions is received in a handover command message from a first cell;

receiving a second configuration of a second set of random access channel occasions of a second cell,

wherein the second set of random access channel occasions is different from the first set of random access channel occasions, and

wherein system information of the second cell includes the second configuration of the second set of random access channel occasions; and

transmitting a random access channel preamble on a random access channel occasion, where at least one selected from the first set of random access channel occasions and the second set of random access channel occasions includes the random access channel occasion,

wherein the first cell is different from the second cell.

2. The method according to claim 1 , wherein the first cell is a serving cell and the second cell is a handover target cell.

3. The method according to claim 1 ,

wherein the random access channel occasion is an available random access channel occasion of the second set of random access channel occasions, and

wherein the method further comprises:

receiving an indication of availability of the available random access channel occasion of the second set of random access channel occasions via a dynamic physical-layer signaling, where the dynamic physical-layer signaling is within a number of slots including a random access channel slot,

wherein the random access channel slot includes at least one random access channel occasion of the second set of random access channel occasions including the random access channel occasion; and

determining the available random access channel occasion in the random access channel slot based on the second configuration of the second set of random access channel occasions and based on the indication of availability of the available random access channel occasion of the second set of random access channel occasions.

4. The method according to claim 3 , wherein the dynamic physical-layer signaling is downlink control information in a group-common physical downlink control channel.

5. The method according to claim 4 , wherein the group common physical downlink control channel is spatially quasi-co-located with at least one of one or more synchronization signal and physical broadcast channel blocks and one or more channel state information-reference signal occasions that are associated with the random access channel occasion.

6. The method according to claim 1 , wherein the first configuration of the first set of random access channel occasions includes information of a set of random access channel slots, where a random access channel slot of the set of random access channel slots includes at least one random access channel occasion of the first set of random access channel occasions.

7. The method according to claim 6 , wherein the first configuration of the first set of random access channel occasions further includes an indication of a number of time domain multiplexed random access channel occasions in each random access channel slot of the set of random access channel slots.

8. The method according to claim 1 , wherein the first configuration of the first set of random access channel occasions includes an indication of a number of frequency domain multiplexed random access channel occasions at a given time instance, where each random access channel occasion is associated with at least one synchronization signal and physical broadcast channel block.

9. The method according to claim 1 ,

wherein the first configuration of the first set of random access channel occasions includes information of a random access channel preamble format,

wherein the random access channel occasion is from the first set of random access channel occasions, and

wherein a duration of the random access channel preamble is determined based on the random access channel preamble format.

10. The method according to claim 1 ,

wherein the second configuration of the second set of random access channel occasions includes a configuration of at least one random access channel preamble format,

wherein the method further comprises receiving information of a random access channel preamble format selected from the at least one random access channel preamble format, where the information of a random access channel preamble format is received via a dynamic physical-layer signaling, and

wherein transmitting comprises transmitting the random access channel preamble on the random access channel occasion according to the received random access channel preamble format.

11. The method according to claim 10 , further comprising:

receiving information of a number of uplink symbols in a random access channel slot, where the random access channel slot includes at least the random access channel occasion; and

selecting a random access channel preamble format from the at least one random access channel preamble format for the random access channel occasion based on the received information of the number of uplink symbols in the random access channel slot,

wherein transmitting comprises transmitting the random access channel preamble on the random access channel occasion in the random access channel slot according to the selected random access channel preamble format.

12. The method according to claim 1 , wherein the first set of random access channel occasions is a subset of the second set of random access channel occasions.

13. The method according to claim 1 , wherein the second set of random access channel occasions is a subset of the first set of random access channel occasions.

14. The method according to claim 1 , wherein the first set of random access channel occasions comprises a first number of random access channel slots and the second set of random access channel occasions comprises a second number of random access channel slots, where the first number of random access channel slots is different from the second number of random access channel slots.

15. The method according to claim 14 , wherein the first number is larger than the second number.

16. An apparatus comprising:

a controller that controls operations of the apparatus; and

a transceiver coupled to the controller, where the transceiver

receives a first configuration of a first set of random access channel occasions, where the first configuration of the first set of random access channel occasions is received in a handover command message from a first cell,

receives a second configuration of a second set of random access channel occasions of a second cell,

wherein the second set of random access channel occasions is different from the first set of random access channel occasions, and

wherein system information of the second cell includes the second configuration of the second set of random access channel occasions, and

transmits a random access channel preamble on a random access channel occasion, where at least one selected from the first set of random access channel occasions and the second set of random access channel occasions includes the random access channel occasion,

wherein the first cell is different from the second cell.

17. The apparatus according to claim 16 , wherein the first cell is a serving cell and the second cell is a handover target cell.

18. The apparatus according to claim 16 ,

wherein the random access channel occasion is an available random access channel occasion of the second set of random access channel occasions, and

wherein the transceiver

receives an indication of availability of the available random access channel occasion of the second set of random access channel occasions via a dynamic physical-layer signaling, where the dynamic physical-layer signaling is within a number of slots including a random access channel slot, where the random access channel slot includes at least one random access channel occasion of the second set of random access channel occasions including the random access channel occasion, and

determines the available random access channel occasion in the random access channel slot based on the second configuration of the second set of random access channel occasions and based on the indication of availability of the available random access channel occasion of the second set of random access channel occasions.

19. The apparatus according to claim 16 , wherein the first configuration of the first set of random access channel occasions includes information of a set of random access channel slots, where a random access channel slot of the set of random access channel slots includes at least one random access channel occasion of the first set of random access channel occasions.

20. The apparatus according to claim 16 , wherein the first configuration of the first set of random access channel occasions includes an indication of a number of frequency domain multiplexed random access channel occasions at a given time instance, where each random access channel occasion is associated with at least one synchronization signal and physical broadcast channel block.